chapter 5 Cycloalkanes

Ring Strain and Stability
• Although the cycloalkanes show many of the same properties and chemistry as the acyclic alkanes, there are two additional features that result from the constraint of binding the chains into a ring: • Ring strain and conformational effect.
cyclobutane cyclopropane cyclopentane cyclohexane
Complex Cycloalkanes
• Naturally occurring materials contain cycloalkane structures • Examples: chrysanthemic acid (cyclopropane), prostaglandins (cyclopentane), steroids (cyclohexanes and cyclopentane)
Stereoisomers
• Compounds with atoms connected in the same order but which differ in three-dimensional orientation, are stereoisomers • The terms “cis” and “trans” should be used to specify stereoisomeric ring structures • Recall that constitutional isomers have atoms connected in different order
bicyclo[4.3.0]nonane
Spiro[4.5]decane
Bicyclic hydrocarbons
one-carbon bridge bridgehead carbons two-carbon bridge three-carbon bridge bicyclo[3.2.1]octane
Heats of formation per-CH2- for the cycloalkanes
n compound ΔHfo/n kcal/mol n 8 compound Cyclooctane ΔHfo/n kcal/mol -3.75
3 cyclopropane 4.25
4 cyclobutane
60o
90o
108o
120o
128.6o
135o
Conformation of Cycloalkane
Conformation of Cyclopropane
• 3-membered ring must have planar structure • Symmetrical with C–C–C bond angles of 60° • Requires that sp3 based bonds are bent (and weakened) – severe angle strain • All C-H bonds are eclipsed – torsional strain
Bicyclic and polycyclic compounds
• If two rings share two or more common atoms, the compound is called a bicyclic compound(桥环化合物). • If two rings have a single common atom, the result is called a spirocyclic compound (螺环化合物).
Conformations of Cyclobutane
• Cyclobutane has less angle strain than cyclopropane but more torsional strain because of its larger number of ring hydrogens • Cyclobutane is slightly bent out of plane - one carbon atom is about 25° above
5,7,7-Trimethyl-bicyclo[2.2.1]hept-2-ene
bicyclo[4.4.0]dec-1(6)-ene
Spirocyclic hydrocarbons
• The numbering of substituted bicyclic ring systems always assigns the 1 to one next to spiro-carbons in smaller ring. • Numbering proceeds around each bridge in turn, beginning with the smaller bridge. • Only after this rule is taken into account are the lowest numbers then assigned to double bonds and substituents
Bent bond
The Nature of Ring Strain
• Rings larger than 3 atoms are not flat • Cyclic molecules can assume nonplanar conformations to minimize angle strain and torsional strain by ring-puckering • Larger rings have many more possible conformations than smaller rings and are more difficult to analyze
bicyclo[4.3.0]nonane
carbon number of whole bicyclic ring carbon number of each bridge according bigger to small
• The numbering of substituted bicyclic ring systems always assigns the 1 to one of the bridgehead carbons. • Numbering proceeds around each bridge in turn, beginning with the largest bridge. • Only after this rule is taken into account are the lowest numbers then assigned to double bonds and substituents.
1.65
9
Cyclononane
-3.55
-4.95
5 cyclopentane -3.65
10 Cyclodecane
6 cyclohexane
-4.95
11 Cycloundecane
12 cyclododecane
-4.65
-4.55
7 cycloheptane -4.05
• Stability of cycloalkanes: 3<4<5<6>7>8>9
菊酸
前列腺素
可的松
Naming Cycloalkanes
• Rules are similar to those for open-chain alkanes • Count the number of carbons in the ring, and add the prefix cyclo- to the name of the corresponding alkane • If a substituent is present on the ring, count the number of carbon atoms in the ring and the number in the largest substituent chain. If the number of carbon atoms in the ring is equal to or greater than the number in the substituent, the compound is named as an alkyl-substituted cycloalkane • For substituted cycloalkane, start at a point of attachment as C1 and number the substituents on the ring. • If there are two or more substituents, begin numbering at the group with alphabetical priority, and give the other substituents the lowest possible number
Chapter 5 Cycloalkanes
Cycloalkanes
• Cycloalkanes are alkanes that have carbon atoms that form a ring (called alicyclic compounds) • Simple cycloalkanes rings of CH2 units, (CH2)n, or CnH2n • Structure is shown as a regular polygon with the number of vertices equal to the number of C’s (a projection of the actual structure)